The biosynthetic and degradation pathways of 5-hydroxytryptamine (5-HT), a putative neurotransmitter, are being characterized. The preparations being emphasized are identified, 5-HT containing neurons in invertebrates, especially the metacerebral cells of the marine mollusc, Aplysia. However, the raphe nuclei of the rat brain are also being studied. Gas chromatography-mass spectrometry (GC-MS) is being utilized as the chief analytical tool in this study. As part of the charaterization of 5-HT biosynthesis, a GC-MS assay for the activity of tryptophan-5-hydroxylase (EC 1.9.9.1.4), the enzyme thought to mediate the rate determining step in 5-HT synthesis, is being developed. This assay should have sensitivity that is at least equal to, and in most cases far superior to, currently employed assays for typtophan-5-hydroxlase activity. The GC-MS assay will be used to characterize the effects of the levels of trypthophan, the biochemical precursor of 5-HT, on the rate of 5-HT biosynthesis. The GC-MS is also being used to characterize the degradation pathways of 5-HT. This is being accomplished utilizing deuterium labeled 5-HT as a tracer, and using the mass spectrometer to detect labeled metabolites of 5-HT.